1. Field of the Invention
This invention relates to spring counterbalance mechanisms suitable for counterbalancing window sashes or the like. The invention relates more particularly to a counterbalance having at least two rotatable drums, spring means within the drums and connected to the drums to generate counterbalancing forces, at least two rotatable reels respectively coupled to the drums by gear teeth on corresponding exterior flanges on the drums and reels, and means for connecting cords to the reels for transmitting the forces to a point of application.
2. Description of Related Art
An example of a prior device is shown in commonly assigned U.S. Patent Application Ser. No. 06/558,174 of Robert J. Zavatkay, filed Dec. 5, 1983, the disclosure of which is incorporated herein by reference. That application discloses a counterbalance mechanism having at least two rotatable cylindrical drums, each having a cord-receiving exterior surface and an interior space surrounded by substantially all of the exterior surface. Backwound prestressed power springs occupy the interior spaces and are connected to the drums to generate counterbalancing forces, which are transmitted by cords wound on the drums. In one embodiment, a first drum and a central arbor integral with a second drum define an annular spring-receiving space. The second drum is radially outward of part of the first drum, and the spring is attached to the arbor and to the interior periphery of the first drum. In a second embodiment, two of the preceding type of assembly are employed in tandem, with a cord being connected to each drum of the first assembly and wound around the corresponding drum of the second assembly for transmitting a force generated by both of the springs.
The device of the '174 application provides a counterbalancing force substantially greater than that of prior devices, while occupying approximately the same space. The dimensional relationships of the springs and the drums provide for enhanced spring power in minimal space.